LTC6603 Linear Technology, LTC6603 Datasheet - Page 15

• Manufacturer Part Number: LTC6603
• Description: Dual Adjustable Lowpass Filter
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
GAIN1 and GAIN0 are the gain control bits (register bits
D6 and D7 when in serial mode). Their function is shown
in Table 1. In serial mode, register bit D1 can be set to
“1” to put the device into a low power shutdown mode.
Register bit D0 is a general purpose output (Pin 21) when
in serial mode.
Table 1. Gain Control
Self-Clocking Operation
The LTC6603 features a unique internal oscillator which sets
the fi lter cutoff frequency using a single external resistor
connected to the R
mined by the following simple formula (see Figure 5):
Note: R
The design is optimized for V+
where the fi lter cutoff frequency error is typically <3%
when a 0.1% external 54.9k resistor is used (any resis-
tor (R
different resistor values and cutoff frequency control set-
tings (LPF1 and LPF0), the lowpass cutoff frequency can
f
CLK
BIAS
GAIN 1
= 247.2MHz • 10k/R
BIAS
0
0
1
1
Figure 5. R
) tolerance, will shift the clock frequency). With
200
175
150
125
100
75
50
25
≤ 200k
10
DESIRED CLOCK FREQUENCY (MHz)
20
BIAS
BIAS
30
vs Desired Clock Frequency
pin. The clock frequency is deter-
40
GAIN 0
BIAS
0
1
0
1
50
A
, V+
60
D
= 3V, f
70
6603 F05
PASSBAND GAIN
80
CLK
(dB)
12
24
= 45MHz,
0
6
be accurately varied from 24.14kHz to 2.5MHz. Table 2
summarizes the cutoff frequencies that can be obtained
with an external resistor (R
the cutoff frequencies scale with the clock frequency. For
example, if LPF1 and LPF0 are both equal to zero, and
R
from 80MHz to 12.36MHz and the cutoff frequency will
be reduced from 156.25kHz to 24.14kHz. The cutoff
frequencies that can be obtained with external resistor
values of 54.9k and 200k are shown in Table 3 and Table 4,
respectively. When the LTC6603 is programmed for the
cutoff frequencies lower than the maximum, the power is
automatically reduced. The power savings at the middle
bandwidth setting (LPF1 = ‘0’, LPF0 = ‘1’), is about 23%,
while the power savings at the lowest bandwidth setting
(LPF1 = ‘0’, LPF0 = ‘0’) is about 60%.
Table 2. Cutoff Frequency Control, R
Table 3. Cutoff Frequency Control, R
Table 4. Cutoff Frequency Control, R
BIAS
is increased from 30.9k to 200k, f
LPF1
LPF1
LPF1
0
0
1
1
0
0
1
1
0
0
1
1
LPF0
LPF0
LPF0
0
1
0
1
0
1
0
1
0
1
0
1
BIAS
) value of 30.9k. Note that
BIAS
BIAS
BIAS
www.DataSheet4U.com
= 30.9k, f
= 54.9k, f
= 200k, f
LOWPASS BW(kHz)
LOWPASS BW(kHz)
LOWPASS BW(kHz)
LTC6603
CLK
CLK
will decrease
CLK
CLK
156.25
351.78
386.25
386.25
87.94
24.14
96.56
2500
2500
1407
1407
625
= 12.36MHz
= 80MHz
= 45MHz
15
6603f